Mesh does not interpolate using full spectrum of colormap

[MathieuMorlighem]

Hello

first, thanks so much for maintaining this package, it is the best! This is more of a feature request I guess. If I use a jet colormap and use mesh to create a plot, I get this:

using GLMakie
import ColorSchemes.jet
Makie.mesh( [0.0 0.0; 1.0 0.0; 1.0 1.0; 0.0 1.0;], [ 1 2 3], shading = false, color = [1;2;3;3], colormap=jet)

But I would expect something like this (interpolate using the full spectrum of the colormap instead of the between the colors of each vertex):

which would be consistent with MATLAB's patch function:

patch('Vertices',[0.0 0.0; 1.0 0.0; 1.0 1.0; 0.0 1.0;],'Faces',[ 1 2 3],'FaceVertexCData',[1;2;3;3],'FaceColor','interp');
colormap jet

As a side note, I am not sure why this does not work:

using GLMakie
import ColorSchemes.jet
Makie.mesh( [0.0 0.0; 1.0 0.0; 1.0 1.0], [ 1 2 3], shading = false, color = [1;2;3;3], colormap=jet)

Thank you for considering this request! Mathieu

[jkrumbiegel]

Hm I don't know why GLMakie does it like this (I would have assumed the behavior you want was the one implemented) but I wanted to add that this is one of the unresolvable differences between GLMakie and CairoMakie. Vector graphics meshes only support one color per vertex as far as I know, so you always get linear interpolation between the corners and it only kind of works if you have many small triangles.

[ffreyer]

I think what GLMakie does is use the values in color to index the colormap and than interpolate those colors. If you generate a mesh with uv coordinates and use the colormap as a texture you get what you want:

using GLMakie, GeometryBasics
import ColorSchemes.jet

ps = [Point2f(0), Point2f(1, 0), Point2f(1)]
faces = [GLTriangleFace(1,2,3)]
uvs = [Point2f(0), Point2f0(0.5, 0), Point2f0(1, 0)]
m = GeometryBasics.Mesh(meta(ps; uv = uvs), faces)

texture = reshape(get(jet, 0:0.01:1), 1, 101)

Makie.mesh(m, color = texture, shading = false)

[MathieuMorlighem]

Thanks @ffreyer, that looks great. Is there an "easy" way to extend that to a mesh that has more than one triangle (and different values at each vertex)?

[ffreyer]

You can have however many vertices and faces you want, The uv's are always per vertex and they get interpolated for each face. The interpolated values are used to sample a color from the texture. Whether it's easy to extend depends on what you want the end result to be. If you want to have colors change in y direction you could probably just do

ys = map(p -> p[2], ps)
ymin, ymax = extrema(ys)
uvs = [Point2f0((y - ymin)/(ymax - ymin), 0) for y in ys]